This invention relates to magnetic bubble propagation circuits and to methods for forming such circuits.
Magnetic domain memories include one or more memory storage loops, accommodating a number of magnetic single wall domains or bubbles, each of which represents one bit of binary information. These bubbles are rotated about the loop in a synchronized and controlled manner so that access can be gained to the stored information. A number of different approaches have been utilized in the formation of these bubble paths or circuits. Typically, in field accessed systems a path or track is established by laying down a pattern of a series of thin film permalloy tiny geometric shapes or circuit elements so that as an in-plane magnetic drive field is rotated the elements are sequentially polarized plus and minus to cause the propagation or step-wise movement of the bubbles along the path. Widely used bubble mover circuit elements are alternating T and bar shapes, alternating Y and bar shapes, and chevron shapes.
While these bubble mover circuit elements function satisfactorily, the small size of these circuit elements used for magnetic bubbles of the typical 5 micron size and the precision with which these elements must be applied, strains the limits of existing masking and photolithographic techniques. Thus at higher bit densities and higher drive speeds, these permalloy alternating circuit element tracks tend to be unreliable and unsatisfactory. In order to increase bit density or information storage capacity per unit wafer or chip area and relative speed as the bubbles are propagated through typical memory registers, attention is being directed to reducing the domain size into the submicron range. This, of course, raises severe difficulties in the masking and photolithography of the circuit element structures to reliably provide precisely shaped and spaced circuit elements for the drive track.
Ion-implanted patterns in garnet films have also been employed (Wolfe et al., AIP Conference Proc. No. 10, 1972, p. 339) for field-access bubble propagation tracks. These patterns avoid the use of small geometry wherein the bubble path is formed of discrete tiny spaced circuit elements of soft magnetic material. Instead gross geometry may be utilized by employing a mask to ion-implant a continuous elongate pattern with a generally serrated or wavy edge along which the domains are propagated. Such gross geometry ion-implanted patterns, however, have certain disadvantages such as producing relatively mild pole strengths when an inplane field is applied which gives rise to smaller propagation margins at high rates of drive field rotation. This leads to failure to have the bubble reliably propagate or to propagate at all.